Airborne radar is used in aircraft navigation to generate a radar map of the ground in the vicinity of the aircraft. In some cases, the antenna aperture area available to an airborne radar device is limited, leading to a relatively large angular width of the anterna's main beam. The width of the beam can result in substantial “smearing” of the map imagery in the cross-range direction, especially in the case of highly reflective targets that are narrow relative to the beam width along the length of the azimuth. In some instances, image smearing can make it difficult for a pilot to identify features on the ground important for the effective use of the imagery.
Signal-processing techniques exist that improve the cross-range resolution of radar ground maps. One commonly used technique uses the gradient in Doppler frequency across the antenna's main beam to sharpen the image. But a disadvantage of this technique is that the gradient in Doppler frequency approaches zero as the pointing direction of the antenna approaches the direction of the velocity vector. Therefore, this technique carries the disadvantage of not being effective in the direction in which the aircraft is travelling.
Another technique for resolution enhancement uses monopulse radar. Processing of monopulse radar signals provides cross-range enhancement independent of direction, but unlike Doppler methods does not provide true resolution improvement. The mono-pulse technique also comes at the cost of significant additional expense and complexity.